13.1 Crystal Structure--- Phase Diagram


Chemical Concepts Demonstrated: The effect of temperature on the structure of a solid; body-centered and face-centered cubic

Demonstration:











    An iron wire is enclosed in a glass vessel (variac) and is purged with N2 or Ar to prevent a reaction with O2.

    A voltage of 6-8 volts is sent through the wire.

Observations:

    When the temperature of the wire exceeds 910 C, the wire sags. When the variac is off, the wire returns to its inital dimensions.

Explanation:

    Iron crystallizes in a body centered cubic structure at room temperature. The density of iron in this phase (7.86 g/cm3) implies an atomic radius of 0.124 nm.   At around 910 C, pure iron changes from body-centered cubic to face-centered cubic packing.  This phase change results is accompanied by a decrease in the density of the iron, and the iron wire sags.  From this decrease in density, it is assumed that the atomic radius of the iron atom at 910 C must be significantly larger than at room temperature.